As offshore exploration and production of oil and gas has moved into deeper water, it has become increasingly important to reduce weight, lower costs, and improve reliability of water-depth sensitive systems such as risers and the like. The term riser generally describes various types of pipes or conduits that extend from the seabed toward the surface of the water. By way of example only, these conduits may be used as drilling risers, production risers, workover risers, catenary risers, production tubing, choke and kill lines, and mud return lines. Conventional risers are normally constructed of various metal alloys such as titanium or steel. More recently, however, the oil and gas industry has considered a variety of alternative riser materials and manufacturing techniques including the use of composite materials.
Composite materials offer a unique set of physical properties including high specific strength and stiffness, resistance to corrosion, high thermal insulation, dampening of vibrations, and excellent fatigue performance. By utilizing these and other inherent physical characteristics of composite materials, it is believed that composite risers may be used to lower system costs and increase reliability of risers used in deep water applications. Although there has been a significant effort in the last decade to facilitate and to increase the general use of composites in offshore applications, the acceptance of composite materials by offshore operators continues to be a relatively slow and gradual process. Reasonably good progress has been made to expand the usage of composites for topside components such as vessels, piping and grating. Some advanced components such as high-pressure riser accumulator bottles have already been used successfully in the field. However, in view of the reduced weight, extended life span, lower cost and other enabling capabilities, composite risers are particularly appealing for deep water drilling and production operations.
Composite risers are generally constructed of a series of joints or sections each having an inner metal liner assembly and a number of structural composite overwrap layers which enclose the metal liner assembly. Typically, a metal liner assembly comprises a thin tubular metal liner, usually of titanium or steel, coaxially secured to a metal connector assembly. The connector assembly includes both a metal-to-composite interface (MCI) and a transition ring. The metal liner is secured to the MCI and the connector assembly through the transition ring. The transition ring can be machined as an integral part of the connector assembly or made separately and then welded to the connector assembly. The connector assembly is a standardized interface at the end of each riser section which facilitates the attachment of one riser section to the next in series using flanges, threaded fasteners or the like. The metal liner and the connector assemblies at each end are then usually enclosed within an elastomeric shear ply, followed by a composite overwrap reinforcement to form a composite riser section. The composite riser section is then heated to cure the elastomeric shear ply and the composite overwrap. The elastomeric shear ply allows a small amount of relative movement between the metal liner assembly and the composite overwrap to accommodate for differences in coefficients of thermal expansion and elastic modulus. An external elastomeric jacket and a further composite overwrap may also be provided over the composite riser section and thermally cured to provide additional impact protection and abrasion resistance in an attempt to limit external damage to the composite riser section.
In application, the metal liner assembly functions to prevent leakage due to the inherent cracking characteristics of the composite material itself. Over time, the matrix in the composite material will tend to develop micro cracks at pressures lower than those at which the composite fibers themselves will fail. The matrix micro cracking is due to the thermal stresses induced by the curing cycle and the mechanical stresses induced during the shop acceptance pressure test of the composite riser section during the manufacturing process. Thus, although the metal liner assembly does not provide a great deal of mechanical strength to the riser, it functions to assure the fluid tightness of the composite riser and to prevent the leakage under conditions of matrix cracking which are inevitable.
The composite overwrap is secured to the metal liner assembly through the metal-to-composite interface (MCI). A traplock MCI may be used to mechanically lock a number of helical (axial) composite plys into a series of annular grooves with several hoop (circumferential) plys of the composite forcing the helical plys downward into the grooves. Accordingly, there is a need for a metal lined composite riser which can offer the benefits of high strength and reduced weight, which has been designed to provide greater field reliability through the use of a traplock MCI that will ensure that the composite material remains firmly adhered to the metal liner assembly throughout the useable lifetime of the riser.